linux-kernel - Re: [PATCH 3/6] x86/entry: Use PUSH_AND_CLEAR

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Date:   Thu, 19 May 2022 19:46:24 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Mark Rutland <mark.rutland@....com>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 3/6] x86/entry: Use PUSH_AND_CLEAR_REGS for compat

On 5/19/22 18:11, Lai Jiangshan wrote:
> On Fri, May 20, 2022 at 1:35 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>>
>> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
>> index ed2be3615b50..f76e674d22c4 100644
>> --- a/arch/x86/entry/entry_64_compat.S
>> +++ b/arch/x86/entry/entry_64_compat.S
>> @@ -200,7 +200,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_safe_stack, SYM_L_GLOBAL)
>>   SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
>>          movl    %eax, %eax              /* discard orig_ax high bits */
>>          pushq   %rax                    /* pt_regs->orig_ax */
>> -       PUSH_AND_CLEAR_REGS rax=$-ENOSYS
>> +       PUSH_AND_CLEAR_REGS rcx=%rbp rax=$-ENOSYS
> 
> Some comments need to be here to explain why %rcx is stashed in %rbp.
> 

I am curious. That comment wasn't needed before the problem fixed here
was introduced. The comment is unrelated to the bug fix. Why is it needed
now, and why would it be a prerequisite for fixing a critical bug ?
Shouldn't that comment be added in a separate patch ?

Thanks,
Guenter